1. Field of the Invention
The invention relates to a selective catalyst for aromatising aliphatic or alicyclic hydrocarbons with at least 7 carbon atoms in the chain, and to methods of its production and use.
2. The Prior Art
Aromatic compounds such as toluene, ethylbenzene , p-xylene and o-xylene are important aromatic hydrocarbons, which are components of high octane fuels and are used as starting materials for a wide variety of synthetic reactions in the chemical industry. Aromatisation thus belongs to the most important processes of petroleum chemistry for the refining of paraffinic hydrocarbons. Because of the indispensability of catalysts for the technical realisation of the aromatisation process, the development of effective catalysts is a constant demand on catalysis research.
The recovery of aromatic compounds from reactions of hydrocarbon fractions containing mainly C.sub.6 -C.sub.8 paraffins has been carried out for decades according to the known reforming process and its modifications. The successfully used Pt/Al.sub.2 O.sub.3 reforming catalysts, which have meanwhile been modified and improved in many ways, e.g. by the use of bi- or multi-metal additives and by variations of the substrate in different ways, are described for example by B. C. Gates, J. R. Kratzer and G. C. Schuit, Chemistry of Catalytic Processes, McGraw-Hill, N.Y. 1979. In general, a yield of 25-40% C.sub.6 -C.sub.8 aromatic compounds is achieved at 450-550.degree. C. and pressures of 1-2 MPa using a hydrogen/carbon molar ratio of 10/1 to 100/1 with catalysts containing 0.5-1.0% by weight of platinum. Side products of the reforming process are hydrogen formed by dehydrogenation reactions and lower paraffins formed by hydrogenolysis (hydrocracking), which reduce the yield of useful materials.
In spite of the progress achieved, the above named catalyst systems have disadvantages. Catalysts containing noble metals are expensive. The temporal stability of the catalysts under the necessary working conditions is worthy of improvement, meaning that additional expense and/or environmental problems occur in the regeneration or processing of the catalysts. The selectivity for the end-product is diminished by the above named side-reactions; the valuable crude oil carbon carrier is exploited inefficiently.